In recent years, transparent thermoplastic resins such as polyarylate resins, polymethyl methacrylate resins, polystyrene resins and polycarbonate resins are preferably used as a molding material in the optical field. In these applications, the size or amount of foreign matter (e.g., dusts, carbides, gelled products, etc.) in the resin, and whether the hue of the resin is good or bad largely influences the quality of the product. In particular, in the manufacture of an optical light guide plate, a change of hue of the resin constituting a substrate and a foreign matter in a substrate give an extremely large influence against the reliability of the product.
In a method of obtaining a molded substrate of a relatively small and medium sized mobile phone or the like by means of injection molding, or a method of manufacturing a sheet molded substrate or film molded substrate by melt extruding a transparent thermoplastic resin and extrusion molding the extruded molten material, it is considered that it is very difficult to reduce gels resulting from the resin.
In particular, in the case where it is intended to manufacture a sheet molded substrate or film molded substrate by using a thermoplastic resin which is liable to generate a gel, a manufacture condition becomes severe for the purpose of decreasing the amount of gels generated in the sheet molded substrate or film molded substrate.
In other words, if a transparent thermoplastic resin pellet which hardly generates gels is used as a raw material for manufacturing a sheet molded substrate or film molded substrate, only a processing condition concentrated on the quality (e.g., uniform wall thickness properties, surface smoothness, etc.) of the sheet substrate or film substrate is enough, and a stable sheet substrate or film substrate is obtained.
In addition, a transparent thermoplastic resin which is used in the optical field is required to be less yellowish.
For that reason, a transparent thermoplastic resin pellet capable of manufacturing a highly transparent sheet molded substrate or film molded substrate with small generation amount of gels is required.
Patent Document 1 discloses a method of obtaining a thermoplastic resin in which the amount of foreign matter is reduced by centrifuging a thermoplastic resin solution and water to separate a resin solution phase and recovering it. In addition, Patent Document 2 discloses a method of reducing the amount of foreign matter by filtering a polycarbonate resin in molten state with a polymer filter. In addition, Patent Document 3 discloses a method of manufacturing a polycarbonate resin pellet with a small amount of foreign matter by cooling a melt-extruded polycarbonate resin by using cooling water having a low electric conductivity and a small amount of foreign matter.
Meanwhile, in order that a melt extruder which is used for the manufacture of a thermoplastic resin pellet may adapt to resins of the same kind but having a different melt viscosity from each other, or thermoplastic resins of a different kind from each other, by one machine, there is often taken a method of dividing a screw of a single-screw or twin-screw melt extruder to several to several tens elements and combining such elements so as to make it possible to change the screw shape in conformity with properties of the respective resins. However, when a screw made of a combination of elements is used, large sized colored materials and foreign matter such as gelled products are often generated.
In contrast, Patent Document 4 discloses a method of producing a transparent thermoplastic resin pellet having an extremely small amount of foreign matter by controlling the surface roughness of a joint part end face of screw elements and the maximum height to fixed ranges or less.
However, according to this technique, if a material having low corrosion resistance is used for the screw, there is a concern that crevice corrosion of the screw is generated due to a stress to generate contamination with foreign matter, resulting in a hindrance to reduction of foreign matter. In addition, a screw noise is generated due to torque variations or screw vibration at the time of resin extrusion depending on the constitution, resulting in not only a hindrance to stable production but metal fatigue or deterioration of the screw.
As a method of dissolving the problems in Patent Document 4, thereby making it possible to prevent contamination with foreign matter from occurring, Patent Document 5 discloses a method of melt extruding a transparent thermoplastic resin by using a melt extruder satisfying with requirements such as the number of elements in the screw, roughness of the element joining part, material quality and screw pitch, and subjecting the extruded molten product to extrusion molding to manufacture a sheet.